The present invention relates generally to an improved electrochemical process for the production of complexes of a zerovalent transition metal and at least one trivalent organic phosphorus ligand.
Zerovalent organo-metal complexes have many known uses in the production of various chemical feedstocks and products. These applications include use as catalysts for a variety of reactions, such as the oligomerization of dienes, the isomerization of alkenes and dienes, the hydrogenation of unsaturated organic compounds, or the co-dimerization of olefins or alkynes with diolefins. These complexes may alternatively be subjected to thermal decomposition to provide a substrate with a metal coating.
A variety of chemical processes for producing zerovalent organo-metal complexes are known in the art. One such method is disclosed in U.S. Pat. No. 3,631,191 to Kane. This process involves contacting divalent nickel with a triaryl phosphite and a reducing metal under reaction conditions. However, many chemical processes have the disadvantage of requiring reducing agents, such as organo-metal complexes, which are relatively difficult to handle. It is in part for this reason that electrochemical processes for producing zerovalent organo-metal complexes have been developed.
Electrochemical processes for the synthesis of zerovalent organo-metal complexes are disclosed by U.S. Pat. No. 3,668,086 to Hughes, U.S. Pat. No. 3,887,441 to Hughes and Fahey and U.S. Pat. No. 3,773,632 to Lehmkuhl, as well as the article "Electrochemical Synthesis of Tris(Tri-o-tolylphosphite)nickel(0)", Corain, Bontempelli, De Nardo and Mazzocchin, Organica Chimica Acta 26 (1978) 37-40. Complexes produced by these methods include tris(tri-o-tolylphosphite)nickel(0), nickel tetra(triphenylphosphine), tris(cyclooctatetraene)dimanganese, tetra(tri-n-butylphosphine)nickel, trans-cyclodecatriene-(1,5,9)-nickel and cyclooctenyl-cobalt-cyclooctadiene-(1,5).
Although known electrochemical processes may be used to synthesize zerovalent organophosphorus transition metal complexes, these processes often result in the transition metal plating onto the cathode. This plating necessitates adding a transition metal recovery step to the process to minimize loss of the metal. More importantly, this plating tends to flake off of the cathode during the process and to circulate in the catholyte as finely divided particles of metal, which contributes to loss of the metal and makes isolation of a pure product relatively difficult, if not impossible. Although prior processes have sought to avoid the occurence of metal flakes in the catholyte by utilizing a large excess of the organic ligand, this may contribute significantly to the cost of the process.
It has been discovered, however, that plating of transition metal onto the cathode and the presence of metal particles in the catholyte may be minimized or avoid by the improvement of the present invention, so that a relatively pure organo-phosphorus/metal complex may be obtained. It has further been discovered that incorporation of the improvement of the present invention into an electrochemical process may result in increased product yields.